Service is a broad category encompassing any value, function, or feature that an app needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.

Angular distinguishes components from services to increase modularity and reusability. By separating a component's view-related functionality from other kinds of processing, you can make your component classes lean and efficient.

Ideally, a component's job is to enable the user experience and nothing more. A component should present properties and methods for data binding, in order to mediate between the view (rendered by the template) and the application logic (which often includes some notion of a model).

A component can delegate certain tasks to services, such as fetching data from the server, validating user input, or logging directly to the console. By defining such processing tasks in an injectable service class, you make those tasks available to any component. You can also make your app more adaptable by injecting different providers of the same kind of service, as appropriate in different circumstances.

Angular doesn't enforce these principles. Angular does help you follow these principles by making it easy to factor your application logic into services and make those services available to components through dependency injection.

Services can depend on other services. For example, here's a HeroService that depends on the Logger service, and also uses BackendService to get heroes. That service in turn might depend on the HttpClient service to fetch heroes asynchronously from a server.

DI is wired into the Angular framework and used everywhere to provide new components with the services or other things they need. Components consume services; that is, you can inject a service into a component, giving the component access to that service class.

The injector is the main mechanism. Angular creates an application-wide injector for you during the bootstrap process, and additional injectors as needed. You don't have to create injectors.

An injector creates dependencies, and maintains a container of dependency instances that it reuses if possible.

A provider is an object that tell an injector how to obtain or create a dependency.

For any dependency that you need in your app, you must register a provider with the app's injector, so that the injector can use the provider to create new instances. For a service, the provider is typically the service class itself.

A dependency doesn't have to be a service—it could be a function, for example, or a value.

When Angular creates a new instance of a component class, it determines which services or other dependencies that component needs by looking at the constructor parameter types. For example, the constructor of HeroListComponent needs HeroService.

When Angular discovers that a component depends on a service, it first checks if the injector has any existing instances of that service. If a requested service instance doesn't yet exist, the injector makes one using the registered provider, and adds it to the injector before returning the service to Angular.

When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.

When you register a provider at the component level, you get a new instance of the service with each new instance of that component. At the component level, register a service provider in the providers property of the @Component() metadata.

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